The present invention relates to the production of metal fibers and more particularly to a method of continuously producing elongated fibers of metal by pyrolysis of the corresponding metal carbonyl. It has long been known that a number of metal carbonyls can be pyrolized in a suitable reaction chamber using a variety of specific techniques to provide as one of the dissociation products fibers of the metal which can be collected and employed in a wide variety of end use applications. In particular, the pyrolysis of nickel carbonyl has long been used as a means to produce nickel metal in a powdered or very short fibrous form.
U.S. Pat. No. 1,759,661 to Muller, et al. discloses the concept of mixing a metal carbonyl with an inert gas or vapor that has been preheated, in order to form spongy metal flakes. The carbonyl is decomposed in the free space in the apparatus to produce dense, spongy flakes. The walls of the apparatus provide heat to the free space to cause the carbonyl to decompose.
U.S. Pat. No. 2,726,951 to Ramsay, et al., produces metal fibers by injecting a metal carbonyl through six nozzles in a header attached to the top of a decomposition column. The fibers are produced by decomposition in a special apparatus and hot flue gasses are utilized to heat the apparatus and thereby cause the carbonyl to decompose.
U.S. Pat. No. 2,884,319 to Fabian et al. employs an inert gas thoroughly mixed with nickel carbonyl to form what are described as elongated nickel particles. The carbonyl is decomposed by means of heat generated by a heating element which is contained in a tube. The tube radiates heat which causes the decomposition to occur. The decomposition takes place in a magnetic field.
In U.S. Pat. No. 3,955,962 to Dawihl, et al. fibers are produced in a magnetic field in an apparatus having heated walls and then withdrawn from the chamber
of the apparatus by means of pistons. The piston has a wire mesh face on which metal fibers are grown. A variable motor moves the piston and the velocity of the piston movement corresponds to the growing velocity of the fibers. Schladitz, U.S. Pat. No. 3,570,829 also uses a plunger to remove nickel fibers from the surface on which they grown in the chamber. As in Dawihl, the fibers are formed in a magnetic field and removed from the apparatus by means of a plunger. A heated surface is used to provide the energy to decompose the carbonyl.
Lambert, et al. U.S. Pat. No. 2,604,442 employs ultraviolet radiation to quicken the reaction that produces metal particles from a metal carbonyl. Small metal particles are formed by pre-decomposing the carbonyl by means of an ultraviolet heat source. It should be noted that Lambert, et al. uses ultraviolet but not infrared heat to decompose carbonyl, and in a portion of the apparatus which is exterior to the main reaction chamber.
There are a number of patents, including O'Neill et al. U.S. Pat. No. 3,409,281, Nichols, U.S. Pat. No. 3,243,173 and Schlecht, et al. U.S. Pat. No. 1,836,732 which disclose some form of recirculation of the heating gas having particles entrained therein back to the top of the reaction chamber. In particular, Nichols teaches that the fine particles are continuously recirculated back into the reaction zone until they reach sufficient size to drop into the lower chamber.
From the foregoing, it will be clear that pyrolysis of nickel carbonyl has long been used as a route to produce nickel powders, and extension work has been done to improve the process of producing nickel metal by pyrolysis of nickel carbonyl. All these prior art methods, however, tended to produce either powdery deposits of nickel or deposits of relatively short metal fibers having length to diameter ratios from the range of about 4 or 5:1 and rarely in excess of 10:1.
It has long been desired to produce mats or "biscuits" of random interlayered relatively long metal fibers, having a diameter of at least 0.5 micron and wherein the bulk of the metal deposit is made up of fibers having a length to diameter ratio in excess of 10:1. Such biscuits would have a number of potentially advantageous end uses, particularly as battery and fuel cell electrodes, in the fabrication of conductive polymer composites, filter elements, coatings for microwave absorption, conductive paints, conductive adhesives, conductive gaskets, and high temperature insulators.